The present invention relates generally to an ink jet printing method and apparatus. More specifically it is related to a drop-on-demand ink jet printing method and apparatus in which the droplet ejection is initiated by a light pulse.
There are two general types of drop-on-demand ink jet printing engines: piezoelectric and thermal (bubble-jet).
The first type is based on the expansion and contraction of a piezoelectric crystal due to an electrical field pulse applied along a certain crystal axis. By means of a lever and membrane this mechanical movement is conveyed to the ink in the ink chamber, thus rapidly raising the pressure in the chamber, causing an ink droplet to eject from the chamber nozzle orifice.
The second type of printing engine consists of an ink chamber with a nozzle and a heating element in thermal contact with the ink in the ink chamber. An electrical current pulse applied to the heating element results in the rapid rise of the ink temperature in the immediate vicinity of the heating element, causing rapid evaporation and bubble generation. The bubble expansion and contraction results in the ejection of an ink droplet from the nozzle orifice.
The following problems are common to both the piezoelectric and thermal ink jet technologies:
a) The ink droplet ejection rate is relatively slow. The rate at which a nozzle can repeatedly eject ink droplets is limited by the piezo-crystal resonance or by the bubble generationxe2x80x94contraction time.
b) The electronic drivers and their wiring make the systems very complicated.
c) It is difficult to build a dense multi-nozzle structure, since the physical dimensions of the actuators limit the direct nozzle pitch. It is extremely difficult to miniaturize a piezo-crystal in order to raise the resonance frequency and at the same time keep the amplitude of the vibrations sufficiently high. Similarly, the more electrical energy needed in the initiating pulse of a bubble-jet engine, the larger the dimensions of the heating element must be.
This last problem is shown in FIG. 1, to which reference is now made, which is an illustration of a prior art nozzle structure. A configuration of nozzles 2, each supplied with ink via an opening 3, and each having an actuator 4 within, is arranged so as to minimize the distance D1, known as the xe2x80x9cdirect nozzle pitchxe2x80x9d, between the centers of the actuators 4. An actuator 4 may be, for example, a piezo-crystal or a heating resistor. Each nozzle 2 is tapered so that its orifice 6 is smaller than its opening 3. The orifices form an orifice array 8. The distance D2 between the orifices 8 is known as the xe2x80x9corifice pitchxe2x80x9d. The current state of the art technology allows placing the actuators at a minimum distance of 200-250 micrometers from one another. The structure forming the orifice array 8 already has a much smaller pitch. For example a print head with a linear array of 1,000 nozzles will have a total length of approximately 200 millimeters, while the length of the orifice array will be only 30 to 50 millimeters.
The following additional problems are specific to bubble-jets:
a) The types of ink that can be used in a bubble-jet engine are limited to those inks whose desired chemical and physical properties do not change when the ink is heated.
b) There is a passivation layer on the heating element electrodes, which protects them from reacting with the ink. The violent process of bubble generation progressively degrades this passivation layer, thereby shortening its lifetime.
c) Special measures for cooling the ink are required.
There is yet another ink jet printing technology, which employs the power of an acoustic wave as an immediate agent for ink droplet ejection. By means of a piezo-crystal or other acoustic generator, a pulse of acoustic waves is generated. These waves, which propagate in the ink volume, are focussed by means of acoustic lenses on the free ink surface or on the nozzle""s orifice. Due to the big difference in the acoustic impedance of the ink and the air, an ink droplet is ejected. These types of printing heads have most of the drawbacks of the piezoelectric ink jet. In addition, the ink droplet ejection is sensitive to the wave focussing. Furthermore, in the case of a free ink surface, parasitic surface waves can cause unwanted ink droplet ejection, or can interfere with desired ink droplet ejection.
It can be seen that the conventional ink jet printing methods have intrinsic drawbacks and can be used only in a limited number of applications. Efforts to improve on conventional ink jet technologies have been directed at achieving denser multi-nozzle structures, higher speed operation, ink type independence and simpler manufacturing.
There have been a number of attempts to solve the problems described above. U.S. Pat. Nos. 3,798,365, 4,463,359, 4,531,138, 4,607,267, 4,723,129 and 4,849,774 and European Patent Applications Nos. EP 0 823 328 A1 and EP 0 858 902 A1 describe variations on the existing ink jet technology. In all of these variations, significant problems remain.
European patent application No. EP 0 816 083 A2 discloses a double chamber bubble-jet engine. The ink chamber and the chamber with the working liquid are separated by a membrane which is thermally conductive and thermally expansive. The bubble is generated in the working chamber by means of an electrically controlled heater. The membrane conveys the pulse pressure generated in the working chamber to the ink chamber, and as a result, a droplet of ink is ejected out of the orifice. Thermal conductivity of the membrane is necessary in order to provide efficient cooling of the working liquid. This method inherits all the problems of the conventional bubble-jet method except for ink type limitation. The requirement for thermal conductivity of the membrane limits the materials and technologies for its production.
In a number of patents such as U.S. Pat. Nos. 4,703,330, 4,751,534, 5,339,101, 4,959,674, 5,121,141, 5,446,485, 5,677,718 and 5,087,930, different types of acoustic ink jet printers and improvements in acoustic wave focussing systems are disclosed. In all these engines the above-mentioned problem with complicated wiring is still present. The physical size and large number of individual acoustic sources limit the density of a multi-nozzle head.
An object of the present invention is to provide an ink jet printing apparatus and method free of the above-mentioned problems of conventional ink jets. The present invention is a practical method for producing high-speed, dense multi-nozzle, simple construction printing heads.
There is provided in accordance with a preferred embodiment of the present invention a print head including a single buffer chamber, a body, and a single ink chamber. The single buffer chamber stores a buffer liquid therein. The body forms one wall of the buffer chamber. The single ink chamber shares the body as a wall. The single ink chamber stores ink therein and has a plurality of orifices on a wall opposite to the body.
There is provided in accordance with another preferred embodiment of the present invention a print head including a single ink chamber, a single buffer chamber, and a body between the ink chamber and the buffer chamber. The ink chamber stores ink therein and has a plurality of orifices. A droplet of the ink exits through a selected one of the orifices in the presence of a directional acoustic wave in the vicinity of the selected orifice. The buffer chamber stores a buffer liquid therein within which the acoustic wave is generated. The body provides acoustic coupling between the ink and the buffer liquid.
Moreover, in accordance with a preferred embodiment of the present invention, the plurality of orifices is arranged in a linear array or a two-dimensional array.
Furthermore, in accordance with a preferred embodiment of the present invention, the body is formed of a material which minimizes attenuation of the acoustic wave.
Additionally, in accordance with a preferred embodiment of the present invention the acoustic wave is generated by absorption of laser light in the buffer liquid.
In accordance with a preferred embodiment of the present invention, a wall of the buffer chamber opposite to the body is an optical element substantially transparent for the laser light.
Moreover, in accordance with a preferred embodiment of the present invention, the optical element is a flat optical window or a microlens array which improves focussing of the laser light into the buffer liquid.
There is provided in accordance with a further preferred embodiment of the present invention a printing device including a laser for generating at least one laser beam, a controller, a print head having a plurality of orifices, and an ink supply for supplying ink to the print head. The controller modulates the at least one modulated laser beam according to image data to be printed. The at least one modulated laser beam selectively generates a directional acoustic wave within the print head, thereby inducing an ink droplet to exit a selected one of the orifices onto a printing substrate.
Moreover, in accordance with a preferred embodiment of the present invention, the printing device is a printing press or an ink-jet printer.
Furthermore, in accordance with a preferred embodiment of the present invention, the laser is a laser diode.
Additionally, in accordance with a preferred embodiment of the present invention, the print head is as described above.
Moreover, in accordance with a preferred embodiment of the present invention, the printing device additionally comprises a scanner for moving the modulated laser beam in a scanning direction such that the modulated laser beam is focussed in the vicinity of the selected orifice.
Furthermore, in accordance with a preferred embodiment of the present invention, the buffer liquid flows in a direction perpendicular to the scanning direction.
Moreover, in accordance with a preferred embodiment of the present invention, the buffer liquid is cooled.
There is provided in accordance with a further preferred embodiment of the present invention a printing method for printing ink upon a printing substrate. The method includes the steps of generating a directional acoustic wave within a print head, propagating the acoustic wave toward a selected orifice of the print head, and inducing a droplet of the ink to exit the selected orifice onto the printing substrate. The directional acoustic wave is generated upon absorption of a laser beam within the print head.
Moreover, in accordance with a preferred embodiment of the present invention, the step of generating occurs within a buffer liquid contained in the print head.
Furthermore, in accordance with a preferred embodiment of the present invention, the step of propagating occurs from the buffer liquid through a body into the ink.